Which of these numbers is prime? ${28,\ 41,\ 45,\ 91,\ 95}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 28 are 1, 2, 4, 7, 14, and 28. The factors of 41 are 1 and 41. The factors of 45 are 1, 3, 5, 9, 15, and 45. The factors of 91 are 1, 7, 13, and 91. The factors of 95 are 1, 5, 19, and 95. Thus, 41 is a prime number.